When a secondary battery is used, it is necessary to know an internal state of the battery, such as a state of charge. The internal state of the battery may, however, not be directly measured but estimated based on an inter-terminal voltage, current, or the like of the battery. In the case of the state of charge, for example, there have been known a current integration method in which the state of charge is calculated by integrating the current and an open circuit voltage method in which the state of charge is estimated based on the inter-terminal voltage obtained under a state in which no current flows through the battery (see, for example, Patent Literature 1 and Patent Literature 2).
However, the current integration method requires the state of charge at an initial time, whereas the open circuit voltage method cannot be used under a state in which the current flows. Accordingly, a method using those methods in combination is required. In Patent Literature 2, for example, there is disclosed a method in which the open circuit voltage method is used under the state in which no current flows and the current integration method is used while the current flows.
In both of the current integration method and the open circuit voltage method, the state of charge is calculated based on battery characteristic parameters representing battery characteristics, but the battery characteristic parameters may contain errors owing to temperature, a manufacturing error, deterioration of the battery, and the like. Those errors cause an error in the estimation of the state of charge. If a plurality of methods are used in combination, there has been a problem in that because the respective methods have their own errors, the estimated value of the internal state may discontinuously change at the time when one of the plurality of methods is switched to another.
Further, in Patent Literature 1, there is disclosed a method in which an adaptive filter is used when a plurality of methods are used in combination. In this method, both of the two methods are constantly used, and the results of the two methods are continuously corrected respectively, to thereby prevent the above-mentioned discontinues change. However, in exchange for this, if the battery characteristic parameters contain errors, this method causes an action contradictory to qualitative properties of the battery. Specifically, for example, the state of charge is displayed as if the state of charge were decreasing during the charging of the battery, or the state of charge changes even under the state in which no current flows. Such an action leads to a problem if a host system using the battery is designed on the premise of the qualitative properties of the battery.
Those problems owe to the errors of the battery characteristic parameters. Thus, in Patent Literature 3, for example, there is disclosed a method of estimating the errors of the battery characteristic parameters. In this method, however, the method of estimating the errors of the parameters itself includes parameters representing change characteristics of the battery characteristic parameters, and hence similar problems occur if the change characteristic parameters of the battery characteristics contain errors.